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(10 August 2020 – JPL) NASA’s Dawn spacecraft gave scientists extraordinary close-up views of the dwarf planet Ceres, which lies in the main asteroid belt between Mars and Jupiter.

By the time the mission ended in October 2018, the orbiter had dipped to less than 22 miles (35 kilometers) above the surface, revealing crisp details of the mysterious bright regions Ceres had become known for.

Scientists had figured out that the bright areas were deposits made mostly of sodium carbonate – a compound of sodium, carbon, and oxygen. They likely came from liquid that percolated up to the surface and evaporated, leaving behind a highly reflective salt crust. But what they hadn’t yet determined was where that liquid came from.

By analyzing data collected near the end of the mission, Dawn scientists have concluded that the liquid came from a deep reservoir of brine, or salt-enriched water. By studying Ceres’ gravity, scientists learned more about the dwarf planet’s internal structure and were able to determine that the brine reservoir is about 25 miles (40 kilometers) deep and hundreds of miles wide.

This mosaic image uses false color to highlight the recently exposed brine, or salty liquids, that were pushed up from a deep reservoir under Ceres’ crust. In this view of a region of Occator Crater, they appear reddish. (courtesy: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA)

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This mosaic of Ceres’ Occator Crater is composed of images NASA’s Dawn mission captured on its second extended mission, in 2018. Bright pits and mounds (foreground) were formed by salty liquid released as Occator’s water-rich floor froze after the crater-forming impact about 20 million years ago. (courtesy: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/USRA/LPI)

Ceres doesn’t benefit from internal heating generated by gravitational interactions with a large planet, as is the case for some of the icy moons of the outer solar system. But the new research, which focuses on Ceres’ 57-mile-wide (92-kilometer-wide) Occator Crater – home to the most extensive bright areas – confirms that Ceres is a water-rich world like these other icy bodies.

The findings, which also reveal the extent of geologic activity in Occator Crater, appear in a special collection of papers published by Nature Astronomy, Nature Geoscience, and Nature Communications on Aug. 10.

“Dawn accomplished far more than we hoped when it embarked on its extraordinary extraterrestrial expedition,” said Mission Director Marc Rayman of NASA’s Jet Propulsion Laboratory in Southern California. “These exciting new discoveries from the end of its long and productive mission are a wonderful tribute to this remarkable interplanetary explorer.”

Solving the Bright Mystery

Long before Dawn arrived at Ceres in 2015, scientists had noticed diffuse bright regions with telescopes, but their nature was unknown. From its close orbit, Dawn captured images of two distinct, highly reflective areas within Occator Crater, which were subsequently named Cerealia Facula and Vinalia Faculae. (“Faculae” means bright areas.)

Scientists knew that micrometeorites frequently pelt the surface of Ceres, roughing it up and leaving debris. Over time, that sort of action should darken these bright areas. So their brightness indicates that they likely are young. Trying to understand the source of the areas, and how the material could be so new, was a main focus of Dawn’s final extended mission, from 2017 to 2018.

The research not only confirmed that the bright regions are young – some less than 2 million years old; it also found that the geologic activity driving these deposits could be ongoing. This conclusion depended on scientists making a key discovery: salt compounds (sodium chloride chemically bound with water and ammonium chloride) concentrated in Cerealia Facula.

On Ceres’ surface, salts bearing water quickly dehydrate, within hundreds of years. But Dawn’s measurements show they still have water, so the fluids must have reached the surface very recently. This is evidence both for the presence of liquid below the region of Occator Crater and ongoing transfer of material from the deep interior to the surface.

The scientists found two main pathways that allow liquids to reach the surface. “For the large deposit at Cerealia Facula, the bulk of the salts were supplied from a slushy area just beneath the surface that was melted by the heat of the impact that formed the crater about 20 million years ago,” said Dawn Principal Investigator Carol Raymond. “The impact heat subsided after a few million years; however, the impact also created large fractures that could reach the deep, long-lived reservoir, allowing brine to continue percolating to the surface.”

Active Geology: Recent and Unusual

In our solar system, icy geologic activity happens mainly on icy moons, where it is driven by their gravitational interactions with their planets. But that’s not the case with the movement of brines to the surface of Ceres, suggesting that other large ice-rich bodies that are not moons could also be active.

Some evidence of recent liquids in Occator Crater comes from the bright deposits, but other clues come from an assortment of interesting conical hills reminiscent of Earth’s pingos – small ice mountains in polar regions formed by frozen pressurized groundwater. Such features have been spotted on Mars, but the discovery of them on Ceres marks the first time they’ve been observed on a dwarf planet.

On a larger scale, scientists were able to map the density of Ceres’ crust structure as a function of depth – a first for an ice-rich planetary body. Using gravity measurements, they found Ceres’ crustal density increases significantly with depth, way beyond the simple effect of pressure. Researchers inferred that at the same time Ceres’ reservoir is freezing, salt and mud are incorporating into the lower part of the crust.

Dawn is the only spacecraft ever to orbit two extraterrestrial destinations – Ceres and the giant asteroid Vesta – thanks to its efficient ion propulsion system. When Dawn used the last of a key fuel, hydrazine, for a system that controls its orientation, it was neither able to point to Earth for communications nor to point its solar arrays at the Sun to produce electrical power. Because Ceres was found to have organic materials on its surface and liquid below the surface, planetary protection rules required Dawn to be placed in a long-duration orbit that will prevent it from impacting the dwarf planet for decades.

JPL, a division of Caltech in Pasadena, California, manages Dawn’s mission for NASA’s Science Mission Directorate in Washington. Dawn is a project of the directorate’s Discovery Program, managed by NASA’s Marshall Space Flight Center in Huntsville, Alabama. JPL is responsible for overall Dawn mission science. Northrop Grumman in Dulles, Virginia, designed and built the spacecraft. The German Aerospace Center, Max Planck Institute for Solar System Research, Italian Space Agency and Italian National Astrophysical Institute are international partners on the mission team.

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Astronomers solve mystery of how planetary nebulae are shaped

Astronomers solve mystery of how planetary nebulae are shaped

(17 September 2020 – Center for Astrophysics | Harvard & Smithsonian) Following extensive observations of stellar winds around cool evolved stars scientists have figured out how planetary nebulae get their mesmerizing shapes.

The findings, published in Science, contradict common consensus, and show that not only are stellar winds aspherical, but they also share similarities with planetary nebulae.

Gallery of stellar winds around cool aging stars, showing a variety of morphologies, including disks, cones, and spirals. The blue color represents material that is coming towards you, red is material that is moving away from you. Image 8, in particular, shows the stellar wind of R Aquilae, which resembles the structure of rose petals. (courtesy: L. Decin, ESO/ALMA)

An international team of astronomers focused their observations on stellar winds—particle flows—around cool red giant stars, also known as asymptotic giant branch (AGB) stars. “AGB stars are cool luminous evolved stars that are in the last stages of evolution just before turning into a planetary nebula,” said Carl Gottlieb, an astronomer at the Center for Astrophysics | Harvard & Smithsonian, and a co-author on the paper. “Through their winds, AGB stars contribute about 85% of the gas and 35% of the dust from stellar sources to the Galactic Interstellar Medium and are the dominant suppliers of pristine building blocks of interstellar material from which planets are ultimately formed.”

Despite being of major interest to astronomers, a large, detailed collection of observational data for the stellar winds surrounding AGB stars—each made using the exact same method—was lacking prior to the study, which resulted in a long-standing scientific misconception: that stellar winds have an overall spherical symmetry. “The lack of such detailed observational data caused us to initially assume that the stellar winds have an overall spherical geometry, much like the stars they surround,” said Gottlieb. “Our new observational data shapes a much different story of individual stars, how they live, and how they die. We now have an unprecedented view of how stars like our Sun will evolve during the last stages of their evolution.”

Observations with the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile revealed something strange: the shape of the stellar winds didn’t conform with scientific consensus. “We noticed these winds are anything but round,” said Professor Leen Decin of KU Leuven University in Belgium, and the lead author on the paper. “Some of them are actually quite similar to planetary nebulae.” The new findings may have a significant impact on calculations of galactic and stellar evolution, most pointedly for the evolution of Sun-like stars. “Our findings change a lot,” said Decin. “Since the complexity of stellar winds was not accounted for in the past, any previous estimate of the mass-loss rate of old stars could be wrong by up to a factor of 10.”

The observations revealed many different shapes, further connecting stellar wind formation to that of planetary nebulae. “The winds we observed exhibit various shapes that are similar to planetary nebulae,” said Gottlieb. “Some are disk-like, while others are shaped like eyes, spiral structures, and even arcs.”

Astronomers quickly realized that the shapes weren’t formed randomly, and that companions—low-mass stars and heavy planets—in the vicinity of the AGB stars were influencing the shapes and patterns. “Just like a spoon that you stir in a cup of coffee with some milk can create a spiral pattern, the companion sucks material towards it as it revolves around the star and shapes the stellar wind,” said Decin. “All of our observations can be explained by the fact that the stars have a companion.”

In addition, the study provides a strong foundation for understanding Sun-like stars and the future of the Sun itself. “In about five billion years, the Sun will become more luminous,” said Gottlieb. “Its radius will expand to a length that is comparable to the current distance between the Sun and Earth, and it will enter the AGB phase.” Decin added, “Jupiter or even Saturn—because they have such a big mass—are going to influence whether the Sun spends its last millennia at the heart of a spiral, a butterfly or any of the other entrancing shapes we see in planetary nebulae today. Our current simulations predict that Jupiter and Saturn will create a weak spiral structure in the wind of the Sun once it is an AGB star.”

About Center for Astrophysics | Harvard & Smithsonian

Headquartered in Cambridge, Mass., the Center for Astrophysics | Harvard & Smithsonian (CfA) is a collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

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Global helium abundance measurements in solar corona

Global helium abundance measurements in solar corona

(18 September 2020 – Naval Research Laboratory) Two U.S. Naval Research Laboratory Space Science Division (SSD) researchers joined an international cadre of scientists July 27 in presenting the results of the first simultaneous global solar corona images of the helium and hydrogen emission that is helping scientists to better understand the space environment.

The paper, “Global Helium Abundance Measurements in the Solar Corona,” was published online in Nature Astronomy and discusses the abundance of helium relative to hydrogen in the solar corona, the outer atmosphere of the sun, seen from earth only during eclipses.

NRL Astrophysicist Dennis Wang, Ph.D., software lead for the HElium Resonance Scattering in the Corona and HEliosphere (HERSCHEL) rocket flight, was responsible for flight and ground software. His NRL colleague, Research Physicist Martin Laming, Ph.D., managed the new model of element abundance fractionation, to include helium.

A composite image of the Sun showing the hydrogen (left) and helium (center and right) in the low corona. The helium at depletion near the equatorial regions is evident. (courtesy: NASA)

“Understanding space weather is important for space situational awareness, that is, forecasting and mitigating the effects of solar activity on Navy and Defense Department satellites,” said Laming. “This was one case where instead of explaining the observations after the fact, I was able to see a prediction I had made come true.”

The HERSCHEL sounding rocket, launched Sep. 14, 2009, provided a number of technological advances in space-based remote sensing. Using a concept developed at NRL for a coronagraph functioning in the extreme ultraviolet regime of the electromagnetic spectrum, the helium coronagraph obtained the first images of the solar atmosphere in the region of the solar wind source surface from light resonantly scattered from helium ions.

The leading model for solar wind variability used by the Department of Defense and National Oceanic and Atmospheric Administration space weather forecasters is an NRL SSD product, known as the Wang, Sheely, Arge Model which is based on simple assumptions about the relation of the solar magnetic field structure and the solar wind, and is reasonably successful in predicting the overall variability of the solar wind as it reaches Earth.

Geomagnetic storms impact radio frequency transmission at frequencies refracted, or reflected, by the ionosphere. The Navy uses magnetic sensors in various battlespace applications, which could be disrupted during large geomagnetic storms and Coronal Mass Ejections. These are major reasons why the Navy is interested in disruptions of the Earth’s magnetic field structure in these measurements.

“There is a long chain of work efforts that go from fundamental understanding of the solar atmosphere, to specifying the observables that need to be monitored before we eventually get to reliable Space Weather forecasts,” said Laming. “In the future, service members should anticipate more reliable satellite-based Command, Control, Communications, Computers, Intelligence, Surveillance, and Reconnaissance.”

Laming demonstrates a strong belief in his model’s prediction capability and his understanding of the sun’s corona adding, “I think we all have more confidence in my model and the conclusions one might draw from it.”

About the U.S. Naval Research Laboratory

NRL is a scientific and engineering command dedicated to research that drives innovative advances for the Navy and Marine Corps from the seafloor to space and in the information domain. NRL headquarters is located in Washington, D.C., with major field sites in Stennis Space Center, Mississippi; Key West, Florida; and Monterey, California, and employs approximately 2,500 civilian scientists, engineers and support personnel.

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hints of fresh ice in northern hemisphere

hints of fresh ice in northern hemisphere

(18 September 2020 – JPL) New composite images made from NASA’s Cassini spacecraft are the most detailed global infrared views ever produced of Saturn’s moon Enceladus. And data used to build those images provides strong evidence that the northern hemisphere of the moon has been resurfaced with ice from its interior.

Cassini’s Visible and Infrared Mapping Spectrometer (VIMS) collected light reflected off Saturn, its rings and its ten major icy moons – light that is visible to humans as well as infrared light. VIMS then separated the light into its various wavelengths, information that tells scientists more about the makeup of the material reflecting it.

The VIMS data, combined with detailed images captured by Cassini’s Imaging Science Subsystem, were used to make the new global spectral map of Enceladus.

In these detailed infrared images of Saturn’s icy moon Enceladus, reddish areas indicate fresh ice that has been deposited on the surface. (courtesy: NASA/JPL-Caltech/University of Arizona/LPG/CNRS/University of Nantes/Space Science Institute)

Cassini scientists discovered in 2005 that Enceladus – which looks like a highly reflective, bright white snowball to the naked eye – shoots out enormous plumes of ice grains and vapor from an ocean that lies under the icy crust. The new spectral map shows that infrared signals clearly correlate with that geologic activity, which is easily seen at the south pole. That’s where the so-called “tiger stripe” gashes blast ice and vapor from the interior ocean.

But some of the same infrared features also appear in the northern hemisphere. That tells scientists not only that the northern area is covered with fresh ice but that the same kind of geologic activity – a resurfacing of the landscape – has occurred in both hemispheres. The resurfacing in the north may be due either to icy jets or to a more gradual movement of ice through fractures in the crust, from the subsurface ocean to the surface.

“The infrared shows us that the surface of the south pole is young, which is not a surprise because we knew about the jets that blast icy material there,” said Gabriel Tobie, VIMS scientist with the University of Nantes in France and co-author of the new research published in Icarus.

“Now, thanks to these infrared eyes, you can go back in time and say that one large region in the northern hemisphere appears also young and was probably active not that long ago, in geologic timelines.”

Managed by NASA’s Jet Propulsion Laboratory in Southern California, Cassini was an orbiter that observed Saturn for more than 13 years before exhausting its fuel supply. The mission plunged it into the planet’s atmosphere in September 2017, in part to protect Enceladus, which has the potential of holding conditions suitable for life, with its ocean likely heated and churned by hydrothermal vents like those on Earth’s ocean floors.

The Cassini-Huygens mission is a cooperative project of NASA, ESA (the European Space Agency) and the Italian Space Agency. JPL, a division of Caltech in Pasadena, manages the mission for NASA’s Science Mission Directorate in Washington. JPL designed, developed and assembled the Cassini orbiter.

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